(a) Field of the Invention
This invention relates to a lubricant composition for the drawing of wire and to the manufacture and use of such composition.
More particularly, the invention is concerned with a lubricant composition which comprises a lubricant intimately mixed with an additive comprising a microencapsulated liquid.
(b) Description of Prior Art
Metal wire of a desired diameter is generally made by drawing a metal wire or rod of larger diameter, which may typically be about 0.2 ins., more usually about 0.218 ins., through a die or series of dies. As the wire is pulled through a die, it undergoes a reduction in diameter. After the wire has been drawn to a smaller diameter wire, the resulting wire is usually redrawn, frequently several times to smaller and smaller diameter. The force required to pull the wire or rod through a die is very high. Furthermore, it is essential that the wire bear a coating of lubricant before it enters the section of the die which produces the reduction in the diameter, otherwise, one or more of the following will occur: the force to draw the wire will be so high that the wire will break, the die will very rapidly wear away, or the die will be badly burned and thereby destroyed, because of the extreme heat generated. The wire is usually drawn continuously at speeds ranging from 500 to 2000 feet per minute.
The nature and the amount of lubricant which should be applied to the wire before it is drawn depend upon such factors as the following: the composition of the metal, the initial diameter of the wire or rod, the extent of reduction in diameter caused by the drawing, the speed of drawing, the geometric design of the die, the metallurgical composition of the die, the use to which the drawn wire will be put, the surface finish of the wire prior to drawing, and the nature of the coating, if any, on the wire prior to drawing.
As the wire is drawn continuously through the lubricant, a small amount of lubricant is deposited on the wire before it enters the die; but, a certain amount is also carried into an elongated conical opening of the die within which it is subjected to friction with the wire and the wall of the elongated opening, as the wire travels through the opening which is of sufficient length and adequate gradual reduction in cross-section to cause the friction to develop a pressure to force the lubricant in quantity between the wire and the die in the deforming or reducing zone of the die. The quantity of lubricant forced into the deforming zone is particularly large if certain types of die called pressure dies are used. These dies and die geometry in general are described in U.S. Pat. Nos. 2,349,652, 3,080,962, 3,641,795 and 3,763,680, the disclosures of which are hereby incorporated herein by reference. Also described in these U.S. patents is the effect of die design on the degree of lubrication, drawing force, temperature of the wire, and stress distribution in the wire.
There are, broadly speaking, two classes of lubricants employed in the drawing of wire: liquids and powders. Liquids are used when drawing fine metal wires and they are frequently used when drawing all sizes of wires made from softer metals, such as aluminum and copper, as opposed to steel. Powders are used when drawing steel wires, except if they are very fine. The higher the carbon content of the steel, the greater the necessity for using a powdered lubricant, because greater force is required to draw high carbon steel wire. Chapter 6 of Volume 1 of the Steel Wire Handbook, edited by A. B. Dove, 1969 published by The Wire Association, Conn., U.S.A., provides a description of many of the different types of wire drawing lubricants. U.S. Pat. Nos. 3,961,511 and 4,168,241 refer to powdered lubricants that are used in drawing wire.
In the case of powdered lubricants, the heat and pressure of the drawing operation transforms the lubricant or, when the wire is coated with a carrier such as lime, the mixture of lubricant and carrier, into a plastic film which serves to reduce substantially the friction of cold work and minimize metal to metal contact.
Usually, the wire or rod prior to drawing has a thin coating of lime, borax or phosphate on its surface, for example, the lime coating is the result of passing the wire through a lime bath to neutralize the residual acid left on it subsequent to pickling and water washing. These coatings act as a carrier for powdered lubricants to an extent which depends on such factors as coating thickness, coating composition, and particle size of the lubricant.
Sometimes, the wire, especially steel wire, includes a coating of a metal on a metal wire core, such as zinc on a steel core in the case of galvanized wire, lead on steel core when making certain types of special wire, including music spring wire, and some rope and bridge wires, or copper on a steel core when drawing very fine wire.
Liquids are not suitable for drawing steel wire, unless the wire is of a low gauge, typically less than 0.018 ins., or the desired reduction in diameter is very small; this is because, the high pressure between the die wall and the wire as the wire enters the deforming or reducing zone of the die completely wipes away the liquid lubricant adhering to the wire, and thereby prevents the lubricant from reducing the frictional forces as the wire passes through the die.
A powdered lubricant is generally not suitable for drawing fine wires or wires composed of softer metals; this is because the use of a powdered lubricant produces a rough surface on the wire subsequent to drawing. Furthermore, lubricant consumption would be very high because of all the powdered lubricant that would be drawn through the die with the wire.
In addition to liquid and powdered lubricants, there are other lubricants intermediate in nature, i.e., pastes and greases. These generally comprise solids dispersed in water or an organic vehicle such as mineral oil.
Most of the liquid lubricants comprise a solution of a water-soluble soap in water along with smaller amounts of additives, such as graphite, corrosion preventatives, antioxidants, sequestering agents, bacteriocides and thickeners.
When a liquid lubricant is employed, the die through which the wire passes is submerged in the liquid and so the wire is surrounded by lubricant as it enters the die.
The solid or powdered lubricants are used in the form of a coarse powder, the average particle size of which is typically about 0.125 ins. in diameter.
On the basis of chemical composition, these powdered lubricants fall into two classes. One class consists of products which are based on sodium soaps. Most of these products comprise approximately 80%, by weight, of a sodium soap with the remainder consisting of various other ingredients, such as borax, lime, and sodium nitrite. The other class contains products which are based on calcium soaps with various amounts of lime fused into them. The lime content determines the thickness of the drawn lubricant film. The thickest film is usually produced with lubricants containing about 75%, by weight, of lime. The fused mixture of lime and calcium soap, generally calcium stearate, usually comprises about 80%, by weight, of the composition with the remainder consisting of mixtures of such products as borax, molybdenum disulphide, sulphur, mica and aluminum stearate.
When a powdered lubricant is employed in drawing wire, it is placed in a box, called a die box. The wire being drawn passes through this box. Where the wire enters the box, there is a hole; but where the wire exits from the box, it passes through a die. Before the wire enters the die, it must pass through the powdered lubricant contained in the box. On doing so, a thin coating of powdered lubricant is deposited on the wire, so that when the wire enters the die it is well lubricated. It is important, therefore, that the powdered lubricant be of such a particle size and particle size distribution that it will tend to build a thin coating of itself on the wire.
It is important that the powdered lubricant have good flow properties; otherwise, as the lubricant is gradually consumed, tunnelling will occur, which will result in the wire not picking up any lubricant. This flowability is determined by such factors as particle size, particle size distribution, and particularly by the basic slipperiness of the surface of the lubricant particles.
Frequently, after the wire drawing operation, it is necessary to anneal the wire and, generally when this is the case, it is desirable that the lubricant be completely or at least almost completely burned off during the anneal. From this standpoint, lubricants which produce a low ash on burning are preferable.
There is generally a dust problem associated with the drawing of wire using a powdered lubricant. This dust is created by the small amount of lubricant still on the wire after its emergence from the die; because, instead of staying on the wire, a large portion of this lubricant flies off the wire in the form of dust. This dust creates a nuisance and in the case of certain lubricants, it can present a hazard to health. This is particularly so when the lime content of the lubricant is high.
If one of the ingredients used in making the powdered lubricant were a liquid which did not chemically react when the lubricant was being made, this liquid would tend to be present on the surface of the particles and, because of surface tension and viscosity effects, its presence would lead to a great reduction in flowability of the powder. The extent to which the flowability was reduced would depend upon such factors as the concentration of the liquid, its solubility in the solid lubricant, its viscosity, and its surface tension.
This limitation on the use of liquid ingredients is unfortunate because there are certain liquids which have properties which would be beneficial. For example, one particular liquid additive which is widely used as an ingredient of liquid lubricants based on mineral oil is zinc dihexyldithiophosphate. This material is very effective as an anti-wear additive. Because wear of the wire drawing die is an important factor in the wire drawing industry, it would be highly desirable to be able to add this material to powdered wire drawing lubricants. Die wear is important because such wear leads eventually to the necessity of replacing the die which in turn entails the cost of a new die and also considerable "down time" and labour.
In addition to the zinc compound described above, which is a very good anti-wear additive, there are other metal dialkyldithiophosphates that provide good anti-wear properties. Some of these compounds are also effective oxidation inhibitors. In this regard reference is made to U.S. Pat. No. 4,066,559.
There are also certain liquid additives which are described as being extreme pressure additives which it would be very beneficial to be able to add to a wire drawing lubricant composition. Sulphurized fatty acid esters are good extreme pressure additives. Many liquid chlorinated oils are effective extreme pressure additives and, hence, provide good lubrication. Apparently, during the drawing operation, the heat generated causes some of the chlorine to be released from the molecule and this chlorine reacts on the surface of the wire to form a compound of unknown composition which promotes lubrication. Analogous mechanisms occur when using other extreme pressure additives. Several liquid fatty acid esters and liquid silicones are also valuable additives. They are good lubricants rather than being anti-wear additives or extreme pressure additives.
Die wear is becoming an increasingly important factor in the economics of wire drawing because a higher proportion of wire or rod is mechanically descaled rather than descaled by acid pickling. Unfortunately, although this change eliminates water pollution, it results usually in a rougher surface on the wire and therefore greater die wear.
Another reason why the limitation regarding the addition of liquids is unfortunate is that the presence of a liquid should reduce the amount of dust generated from the wire as it leaves the die. In particular the liquid would tend to bind the particles of lubricant together on the surface of the wire and thereby reduce the number that leave the surface to form dust.
A problem also exists if it is desirable to add certain liquid ingredients to the liquid, water based lubricants that are employed in drawing of fine wires. Some of these liquid additives cannot be employed because it is difficult to form a stable emulsion of the additive in water. If the liquid ingredient is not present in the entire liquid lubricant in a uniform concentration, but instead is concentrated at the surface of the reservoir, then of course very little, if any, will find its way to the surface of the wire.